To delineate the molecular basis of the polymorphism of human histocompatibility antigens, our panel of monoclonal antibodies will be used to isolate HLA-DR antigens by antibody affinity chromatography. The molecular structure of these antigens will be determined by tryptic peptide maps and amino acid sequence analyses. The monoclonal anti HLA-DR antibodies will also be used to identify precursor polypeptides of HLA-DR antigens in cell free translation directed by poly (A)-RNA siolated from B lymphoid cells expressing HLA-DR antigens. Presently ongoing efforts will be continued to delineate the detailed molecular structure of HLA-DR antigens using DNA cloning techniques and nucleic acid sequencing methods.